You need to reach lvl40+ 
Have enough gems or have units to add for less fee (gems) (transaction fee for it is 6k gems) 
I have a trick that can help bring the trade tax to zero Although it is a bit confusing, as long as you prepare 1 Unit Evolved or 2 Unevolved Units or 3 Mythical Skins, you will not have to lose any tax set by the game and of course your Unit or Skin will not disappear. 
With time zone GMT +0 
I will be online at 2:00 PM - 6:00 AM